Resistance to endocrine/tamoxifen (TAM) therapy is a major clinical challenge for the treatment of breast cancer with as many as 30% to 50% of patients with estrogen receptor alpha-positive (ERa+) breast tumors now displaying intrinsic resistance. The molecular mechanisms underlying TAM resistance (TAM-R) are yet not fully clarified, however, four basic mechanisms appear to be consistently involved to greater or lesser degrees in the development of TAM-R in breast cancer including: (1) the aberrant activation of growth factor signaling cascades that provide compensatory proliferative and survival signals, limiting the efficacy of TAM, (2) tumors responding to TAM as an ERa agonist, (3) the loss of ERa in some TAM-R breast tumors, and (4) the reduced or inhibition of metabolism of TAM to its active metabolites 4OH-TAM and/or Endoxifen. A key feature seen in most TAM-R breast tumors is the constitutive activation of kinases ERK1/2, AKT, PKA, PKCa, and SRC. We recently reported a direct link between circadian disruption by exposure to dim light-at-night (dLEN), its suppression of nocturnal circadian melatonin (MLT), enhanced phosphorylation/activation of these same kinases seen in TAM-R breast cancer as well as the ERa, and the development of intrinsic TAM-R in human breast tumor xenograft growth in female nude rats. LEN is a well-recognized environmental disruptor of the central circadian timing system located in the suprachiasmatic nucleus (SCN) of the brain. The SCN is responsible for the optimal timing of the physiology and metabolism in cells, tissues, and organs that are synchronized by the daily light/dark cycle. The pineal hormone MLT is a key circadian output signal of the SCN (circadian clock) whose nocturnal synthesis is suppressed by dLEN. MLT has been shown to inhibit cellular, molecular, and metabolic processes governing breast cancer initiation, promotion and progression, and epidemiologic studies show that women chronically working night shifts are particularly vulnerable to exposure to LEN-induced circadian disruption and have a significantly increased breast cancer risk. It is unknown if LEN-induced circadian disruption of the nocturnal MLT signal in women receiving endocrine therapy for breast cancer compromises the effectiveness of their therapy. Therefore, the overriding hypothesis of the project is that dLEN will drive the progression of ERa+ breast tumor xenografts in female nude rats to develop complete intrinsic TAM-R by elevating the expression and/or phospho- activation of key kinases and transcription factors including the ERa, by inhibiting the metabolism of TAM to more active metabolites (4OH-TAM and Endoxifen), and by inducing the expression of ABC pumps to cause 4OH-TAM and Endoxifen efflux from tumor cells. The Specific Aims of this project are: 1) to determine if dLEN via circadian MLT disruption/suppression drives intrinsic TAM-R in multiple breast cancer xenograft models and if key mechanisms and signaling pathways are involved, 2) to determine the mechanism(s) by which dLEN and MLT differentially regulate estrogen- and TAM-induced ERa phosphorylation and transactivation, and if dLEN alters the response of ERa to TAM, or drives the loss of ERa, and 3) to assess if administration of TAM or 4OH-TAM at different circadian time points relative to the nocturnal rise in MLT, impacts the metabolism of TAM to its active metabolites, to regulate the overall efficacy and synergy with MLT to induce tumor xenograft regression. If these studies support our hypothesis the implications would be far-reaching and would suggest that breast cancer patients subjected to LEN- induced circadian MLT disruption will be at higher risk for rapidly developing resistance to hormonal therapy and very aggressive breast cancer.